Electric power tools such as a power drill or a hammer drill are sometimes mounted with an attachment device such as a dust collecting device that collects and gathers powder dust produced from a worked material during a drilling operation, or a light unit that illuminates a tip of a tool. When the attachment device is used, it is necessary to supply a driving source to the attachment device. For example, Patent Literature 1 discloses a system, in which an electric power tool mounted with a battery pack as a power source is mounted with a dust collecting device as an attachment device. In this system, an interposing unit is provided on the dust collecting device such that a top face of the interposing unit is fitted to a battery pack mounting unit of the electric power tool and a lower face of the interposing unit is mountable with the battery pack. By mounting the battery pack through the interposing unit to the electric power tool, a power source for the dust collecting device is obtained from the battery pack. Furthermore, Patent Literature 2 discloses a system, in which by connecting a dust collecting device that is attached to a battery pack attaching unit of an electric power tool to an external power source, a power source is supplied from the external power source to the electric power tool and the dust collecting device.